Is it isolated to the sub amps?
Yes.

Have you tried detaching the inputs to the PPI amp?
Yes. The noise goes away completely when I do that.

Try inserting muting plugs to the inputs of the PPI
Did one better than that -- swapped in a different amp. Same symptoms.

We pretty much narrowed the problem down to either defective RCA cables between the "Y"s and the amplifier; or a problem with the remote gain potentiometer for the subwoofer amplifier.

Turns out it was a little bit of both -- one of the solder joints connecting one of the signal wires to the terminal on the pot (don't remember if it was left or right -- the wire was white, if that means anything) had broken and the wire was making only a tentative connection, close enough that it might actually have been arcing, causing the warbling sound. Took an hour to find it, 10 seconds to fix it. The reason it wasn't obvious just listening to the subs and saying, hey, no right (or left?) channel -- the subs are running bridged mono at the amp, so if either channel makes it to the amp, both subs play.

ShoWagon Stereo Lives!!

tanstaafl.